1. Field of the Invention
The present invention relates to a method for producing a flame retardant polycarbonate resin composition. More particularly, the present invention is concerned with a method for producing a flame retardant polycarbonate resin composition by using a screw extruder, which comprises extruding a resin component (mainly comprising a polycarbonate) and an organic phosphorus compound having an acid value of not more than 0.1 mg KOH/g while kneading, wherein the temperature of the resin component in the extruder is not higher than 300xc2x0 C., and the residence time of the resin component in the extruder is not more than 40 seconds. By the method of the present invention, it has become possible to produce a flame retardant polycarbonate resin composition containing an organic phosphorus compound as a flame retardant, in which not only a lowering of the mechanical properties but also a deterioration of the color can be greatly suppressed even under high temperature and high humidity conditions.
2. Prior Art
A resin composition comprising a polycarbonate (PC), an organic phosphorus compound-containing flame retardant and optionally an ABS (acrylonitrile-butadiene-styrene) resin is a flame retardant resin composition which is free from a chlorine/iodide flame retardant. Such a flame retardant resin composition is excellent in various properties, such as melt fluidity, rigidity, impact resistance, heat resistance and resistance to discoloration under light, and hence, it has been widely used as a material for housings in electric appliances and office automation machines, such as computers, printers, word processors and copying machines.
However, in the case of a flame retardant polycarbonate resin composition containing an organic phosphorus compound-containing flame retardant (hereinafter, frequently referred to simply as a xe2x80x9cflame retardant polycarbonate resin compositionxe2x80x9d), the organic phosphorus compound present in the resin composition is likely to suffer heat decomposition or hydrolysis to thereby lower the properties of the resin composition. Particularly, such a resin composition has a problem in that the mechanical properties (such as impact resistance and break strength) and color of the resin composition are markedly lowered under high temperature and high humidity conditions. When it is intended to use the flame retardant polycarbonate resin composition in fields where the resin composition is required to retain the desired properties for a long time, the above-mentioned problem reduces the reliability of the flame retardant polycarbonate resin composition. Therefore, there has been a strong demand to improve the flame retardant polycarbonate resin composition with respect to the resistance thereof to high temperature and high humidity conditions.
Heretofore, various proposals have been made to solve the above-mentioned problem. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 9-188808 (corresponding to EP 711 851) discloses a flame retardant polycarbonate resin composition containing a rubber-grafted copolymer, in which it is attempted to suppress a lowering of the Izod impact strength under high temperature and high humidity conditions by increasing the content of the rubber-grafted copolymer in the resin composition. However, by the technique of this patent document, it is impossible to suppress satisfactorily a lowering of the Izod impact strength of the resin composition under high temperature and high humidity conditions. Further, the technique of this patent document has a defect in that, in accordance with the increase in the rubber content of the resin composition, the properties (such as rigidity, melt fluidity and flame retardancy) of the resin composition are lowered. In Unexamined Japanese Patent Application Laid-Open Specification No. 11-310695 (corresponding to EP 936 243), an organic phosphorus compound having a low acid value of less than 1 mg KOH/g is used as a flame retardant for a polycarbonate resin. However, even when such an organic phosphorus compound having a low acid value is used, it is difficult to obtain a flame retardant polycarbonate resin composition which has a satisfactorily improved resistance to high temperatures and high humidity conditions. In Unexamined Japanese Patent Application Laid-Open Specification No. 11-189714 (corresponding to EP 909 790), it is attempted to obtain a flame retardant polycarbonate resin composition having an improved resistance to high temperatures and high humidity by incorporating into the resin composition an acid scavenger for removing acidic substances formed by the decomposition of a phosphorus compound. However, in the technique of this patent document, it is difficult to control appropriately the amount of the acid scavenger incorporated into the resin composition. Even if the amount of the acid scavenger incorporated into the resin composition is only slightly larger than the appropriate amount, disadvantages are likely to occur such that the melt fluidity of the resin composition is lowered, and that the impact resistance of the resin composition is lowered.
Further, Unexamined Japanese Patent Application Laid-Open specification Nos. 11-246721, 2000-109670 (each corresponding to U.S. Pat. No. 6,177,492) and 2000-95935 disclose a flame retardant polycarbonate resin composition obtained by blending a polycarbonate resin with a styrene polymer resin, a polyphenylene ether resin, a saturated polyester resin, and an organic phosphorus compound-containing flame retardant. However, the resin composition disclosed in these patent documents is unsatisfactory with respect to the resistance to discoloration under light and the impact resistance. Each of these patent documents describes that the kneading time for producing the resin composition is preferably 20 minutes or less. For example, in the working examples of the above-mentioned Unexamined Japanese Patent Application Laid-Open specification Nos. 11-246721, the kneading is conducted for 100 seconds. Each of the above patent documents has no description regarding the resistance of the resin composition to high temperature and high humidity conditions. Needless to say, each of the above patent document has no teaching or suggestion about a method for improving a flame retardant polycarbonate resin composition with respect to the resistance to high temperatures and high humidity.
Thus, although it has been strongly desired to improve a flame retardant polycarbonate resin composition containing an organic phosphorus compound-containing flame retardant with respect to the resistance to high temperature and high humidity conditions, a flame retardant polycarbonate resin composition having satisfactorily improved resistance to high temperature and high humidity conditions has not been realized.
In this situation, the present inventors have made extensive and intensive studies toward developing a method for producing a flame retardant polycarbonate resin composition containing an organic phosphorus compound as a flame retardant, in which not only a reduction in the mechanical properties but also a deterioration of the color can be greatly suppressed even under high temperature and high humidity conditions. As a result, it has unexpectedly been found that, by a method comprising extruding, in a screw extruder, a resin component (mainly comprising a polycarbonate) and an organic phosphorus compound having an acid value of not more than 0.1 mg KOH/g while kneading, wherein the temperature of the resin component in the extruder is not higher than 300xc2x0 C., and the residence time of the resin component in the extruder is not more than 40 seconds, it becomes possible to produce efficiently the above-mentioned excellent flame retardant polycarbonate resin composition. The present invention has been completed, based on this novel finding.
Accordingly, it is a primary object of the present invention to provide a method for efficiently producing a flame retardant polycarbonate resin composition containing an organic phosphorus compound as a flame retardant, in which not only a lowering of the mechanical properties but also a deterioration of the color can be greatly suppressed even under high temperature and high humidity conditions.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawings.